The present invention relates to an apparatus for reading from and/or writing to optical recording media on which data markings are arranged along tracks arranged next to one another. An apparatus of this type has a light source for producing a scanning beam, a focusing means for focusing the scanning beam onto a track of the recording medium, and an intensity distribution converter for reducing the spatial extent of the focus spot of the scanning beam which is produced on the recording medium by the focusing means. Consequently, a particularly good resolution of the focus spot is achieved by means of the aforementioned intensity distribution converter; the method employed is referred to as the superresolution technique.
Such an apparatus employing the superresolution technique is disclosed for example in U.S. Pat. No. 5,121,378. Advantages of the superresolution technique reside in the fact that a smaller extent of the focused light beam can be obtained and data markings of a smaller size can thus be scanned. A smaller size of the data markings permits denser packing and thus a higher information density. A disadvantage of the superresolution technique resides in the fact that secondary beams of increased intensity are produced in addition to the actual light beam. Therefore, a light corona or a similar light distribution is produced adjacent to the light spot of the primary beam on the recording medium, caused by the secondary beams. This light distribution caused by the secondary beams has a disturbing effect on the information transmitted by means of the primary beam. The smaller the extent of the focus spot of the primary beam, the smaller the intensity of the latter becomes and the larger the intensity of the secondary beams becomes.
An object of the present invention is to propose an apparatus in which both the extent of the focus spot of the primary beam is reduced and the disadvantages occurring due to the secondary beams are reduced.
This object is achieved by means of the measures specified in the claims.
The invention provides for the intensity distribution converter to be a filtering strip arranged in the region of the focusing means. This has the advantage of reducing the extent of the central beam, the primary beam, in a perpendicular direction with regard to the strip, which means that a track having a smaller width can nevertheless be scanned without the focus spot of the primary beam overlapping adjacent tracks. The resulting plus/minus first- and higher-order secondary beams produce focus spots in the focal plane, but no large-area light distribution on the recording medium. Disturbing effects caused by the secondary beams can thus be masked out in a targeted manner. The filtering strip is an opaque strip in the simplest case; the following are likewise advantageously possible: a reflective strip, a strip which couples the light out of the optical axis to a great extent in some other way, a frequency-selective filter or a similar component which fulfils a corresponding function.
The invention furthermore provides a detector arrangement having a plurality of detector areas onto which the light reflected from the recording medium is directed, the primary beam and the secondary beams being directed onto different detector areas. This has the advantage that the primary beam can be evaluated separately, that is to say no adverse effects due to secondary beams influenced by other areas of the recording medium occur. According to a variant of the invention, the other detector areas onto which the secondary beams fall are not evaluated; they are then advantageously shadowed or non-photosensitive areas.
According to the invention, the filtering strip is arranged at a tilt angle of greater than zero degrees with respect to the direction of the tracks of the recording medium. Although the filtering strip reduces the extent of the focus spot in the direction perpendicular to the strip, it does not do so in the direction of the strip. Consequently, given parallel orientation of the strip and track to be read, the resolution of the data markings in the track direction is not improved, only that perpendicularly thereto. One advantage of the tilted arrangement of the strip resides in the fact that a compromise between reducing the track width and reducing the length of the data markings can be attained by virtue of the inclination with regard to the tracks. Overall, the readable storage capacity of the recording medium is again increased, given the same wavelength of the light used.
In this case, the tilt angle is advantageously chosen such that the plus/minus first-order secondary beams fall onto adjacent tracks. This has the advantage that, at the same time as the track scanned by the primary beam, the tracks which are adjacent to this track and are swept over by the first-order secondary beams are also evaluated with regard to their information content. This increases the effective read-out speed, that is to say the read-out data rate. In this case, the solution according to the invention is not restricted only to the directly adjacent tracks, rather, depending on the property of the strip, such as width for example, property of the light used, such as the wavelength thereof for example, and properties of the optical elements used and of the recording medium, such as the track spacing for example, provision is also made for scanning, and thus evaluating, the tracks which are in each case the second adjacent tracks, third adjacent tracks or even further removed adjacent tracks by means of the first-order secondary beams. This has the further advantage that intermediate jumps of the scanning beam to adjacent tracks situated at a further remove are necessary less often for a read-out which is as continuous as possible. The invention is likewise not restricted to the use of first-order secondary beams; it likewise lies within the scope of the invention to use the second- or higher-order secondary beams as well. Depending on the configuration of the strip, the intensity of the secondary beams is relatively highly adjustable in comparison with that of the primary beam, so that a good read-out quality can also be obtained for the secondary beams.
According to a further variant of the invention, the tilt angle is chosen such that the secondary beams produced by the filtering strip fall onto track edges. These may be either the left and right edges of the track scanned by the primary beam or edges of the tracks which are adjacent directly or at a remove with respect to the track scanned by the primary beam. One advantage of this variant resides in the fact that the signals of the secondary beams can be used for the known three-beam tracking method. This holds even when the intensity of the secondary beams is low. A smaller possible track width of the recording medium is nevertheless obtained by the reduced extent of the focus spot of the primary beam.
The filtering strip is advantageously arranged in the pupil plane of the focusing means. This has the advantage that disturbances that may be caused by diffraction effects if the filtering strip were at a relatively large distance from the lens and hence from the pupil plane do not occur.
The apparatus according to the invention advantageously has a further focusing means and a gap, which is arranged in the focal plane of the said further focusing means and on whose side remote from the focusing means one of the detector areas is arranged. This has the advantage that it is possible to obtain uncomplicated yet effective separation of primary beam and secondary beams which do not impinge on the gap and are thus shadowed. In this case, the orientation of the gap advantageously corresponds to the direction of the filtering strip. In this case, the largest spatial extent direction of the focus spot of the primary beam coincides with the longitudinal direction of the gap and the smallest spatial extent coincides with the width of the gap. The shadowing effect of the gap is thus optimal.
One of the detector areas advantageously has a dark line area, whose width is greater than the smallest width of the focused primary beam. This has the advantage of enabling detection exclusively of the secondary beams by masking out the primary beam.
In accordance with an advantageous refinement of the invention, the gap arranged before one detector area is formed by the dark line area of the other detector. In this case, the dark line area is of transparent design or is itself a gap. One advantage of this embodiment resides in the compact detector arrangement which is obtained by the combination of the detector area provided for primary beam and that provided for the secondary beams. In this case, the invention is not restricted to the utilization of the first order secondary beams; the higher-order secondary beams are also correspondingly evaluated. To that end, it is then necessary to choose a correspondingly larger width of gap or dark line area in order that the respective lower-order beams are masked out or allowed to pass. A plurality of such detector areas provided with gaps then form a detector consisting of stacked detector areas.
The invention provides for an element which splits the beam cross-section to be arranged in the beam reflected from the recording medium. In this case, the splitting is effected in a similar manner to the shadowing used in the known knife-edge method, with the difference that the area which is shadowed in accordance with the knife-edge method is not shadowed in accordance with the invention, but rather is split and utilized further. Such an element is, for example, a double prism, a grating having areas with different grating constants, a holographic optical element or a similar element which performs the corresponding function. One advantage of this variant resides in the fact that the beam is split between different detector areas and, at the same time, a beam shape which is necessary for the knife-edge focus method is produced. A conventional beam splitter and a shadowing element required for the knife-edge method are thus not necessary; a compact arrangement is obtained.
The filtering strip is advantageously arranged directly on a converging lens. In this case, the converging lens is preferably the focusing lens of the focusing means. The strip is bonded on, vapour-deposited on, cast or formed into the lens during the production thereof, or else worked, for example milled, into the said lens. One advantage of this refinement of the invention resides in the fact that an additional component carrying the filtering strip is not necessary, as a result of which the production is simplified and an outlay in terms of adjustment is not necessary.
Further advantages of the invention are contained in the following description of advantageous configurations of the invention. It goes without saying that the invention is not solely restricted to the variants specified here, but rather includes modifications and supplementations which are familiar or obvious to a person skilled in the art.